⚡ Quick Fix (TL;DR)

The Culprit: N/A – this is a deliberate style choice, not something to fix.

The Fix: Round just the top two corners, leaving the bottom edge square. The four values go top-left, top-right, bottom-right, bottom-left, in that order.

.my-element {
  border-radius: 16px 16px 0 0;
}